Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae. Influenza or flu viruses infect birds and mammals. Three of the five genera of Orthomyxoviridae are influenza viruses: Influenza A, Influenza B and Influenza C. Of these, Influenza A is the most common.
Influenza is typically transmitted through the air in aerosols produced by coughs or sneezes and by direct contact with body fluids containing the virus or contaminated surfaces. Seasonal epidemics of influenza occur worldwide and result in hundreds of thousands of deaths annually. In some years, pandemics occur and cause millions of deaths. In addition, livestock, particularly poultry and swine, are also susceptible to annual epidemics and occasional pandemics which cause large numbers of animal deaths and monetary losses.
Structurally, influenza viruses are similar, having generally spherical or filamentous virus particles of about 80-120 nm made up of similar molecular component. A central core comprising viral proteins and viral RNA is covered by a viral envelope made up of two different glycoproteins and a lipid coat derived from the cell that the viral particle is produced in. Two additional different glycoproteins are anchored within the viral envelope and include portions which project outward on the surface.
The influenza virus RNA genome is typically provided as eight different single stranded, negative sense RNA segments that together make up the genome's eleven viral genes which encode the eleven proteins (HA, NA, NP, M1, M2, NS1, NEP, PA, PB1, PB1-F2, PB2). The eight RNA segments are: 1) HA, which encodes hemagglutinin (about 500 molecules of hemagglutinin are needed to make one virion); 2) NA, which encodes neuraminidase (about 100 molecules of neuraminidase are needed to make one virion); 3) NP, which encodes nucleoprotein; 4) M, which encodes two matrix proteins (the M1 and the M2) by using different reading frames from the same RNA segment (about 3000 matrix protein molecules are needed to make one virion); 5) NS, which encodes two distinct non-structural proteins (NS1 and NEP) by using different reading frames from the same RNA segment; 6) PA, which encodes an RNA polymerase; 7) PB1, which encodes an RNA polymerase and PB1-F2 protein (induces apoptosis) by using different reading frames from the same RNA segment; and 8) PB2, which encodes an RNA polymerase.
Of these eleven proteins, hemagglutinin (HA) and neuraminidase (NA) are two large glycoproteins anchored in the viral envelope and present on the outer surface of the viral particles. These proteins serve as immunogens for immune responses against influenza. HA, which is a lectin that mediates binding of the virus to target cells and entry of the viral genome into the target cell, is expressed as a single gene product, HAD, and later processed by host proteases to produce two subunits, HA1 and HA2, which together form a complex on the surface of influenza viral particles. NA is involved in the release of newly produced mature viral particles produced in infected cells.
There are sixteen known HA serotypes and nine known NA serotypes for Influenza A viruses. The identity of the different serotypes present in a viral particle typically is used to describe a virus. For example, H1N1 is an influenza virus with HA serotype H1 and NA serotype N1; H5N1 is an influenza virus with HA serotype H5 and NA serotype N1. Only H1, H2 and H3 serotypes, and N1 and N2 serotypes usually infect humans.
Influenza strains are generally species or genus specific; i.e. an influenza strain which can infect pigs (a swine influenza virus) typically does not infect humans or birds; an influenza strain which can infect birds (an avian influenza virus) does not infect humans or pigs; and an influenza strain which can infect humans (a human influenza virus) does not infect birds or pigs. Influenza strains, however, can mutate and become infective from one species to another. For example, a strain which only infects pigs, a swine influenza, can mutate or recombine to become a strain that can infect humans only or both pigs and humans. A flu virus commonly referred to as “swine flu” is an influenza virus strain, such as an H1N1 strain, which can infect humans and which was derived from a strain that was previously specific for pigs (i.e. a swine flu virus is a swine origin human influenza or swine derived human influenza). A flu virus commonly referred to as “bird flu” is an influenza virus strain, such as an H5N1 strain, which can infect humans and which was derived from a strain that was previously specific for birds (i.e. a bird flu virus avian origin human influenza or avian derived human influenza).
Vaccinations against influenza are provided seasonally to many humans in developed countries and sometime to livestock. The vaccines used are limited in their protective results because the immune responses induced by the vaccines are specific for certain subtypes of virus. Different influenza vaccines are developed and administered annually based upon international surveillance and scientists' estimations of which types and strains of viruses will circulate in a given year. The virus changes significantly by mutation, recombination and reassortment of the segments. Thus, vaccines given in one year are not considered protective against the seasonal strains that are widely transmitted the following year.
The “flu shot” commonly promoted U.S. Centers for Disease Control and Prevention usually contains three killed/inactivated influenza viruses: one A (H3N2) virus, one A (H1N1) virus, and one B virus. Thus, it is apparent that vaccinations are limited to predictions of subtypes, and the availability of a specific vaccine to that subtype.
The direct administration of nucleic acid sequences to vaccinate against animal and human diseases has been studied and much effort has focused on effective and efficient means of nucleic acid delivery in order to yield necessary expression of the desired antigens, resulting immunogenic response and ultimately the success of this technique.
DNA vaccines have many conceptual advantages over more traditional vaccination methods, such as live attenuated viruses and recombinant protein-based vaccines. DNA vaccines are safe, stable, easily produced, and well tolerated in humans with preclinical trials indicating little evidence of plasmid integration [Martin, T., et al., Plasmid DNA malaria vaccine: the potential for genomic integration after intramuscular injection. Hum Gene Ther, 1999. 10(5): p. 759-68; Nichols, W. W., et al., Potential DNA vaccine integration into host cell genome. Ann N Y Acad Sci, 1995. 772: p. 30-9]. In addition, DNA vaccines are well suited for repeated administration due to the fact that efficacy of the vaccine is not influenced by pre-existing antibody titers to the vector [Chattergoon, M., J. Boyer, and D. B. Weiner, Genetic immunization: a new era in vaccines and immune therapeutics. FASEB J, 1997. 11(10): p. 753-63]. However, one major obstacle for the clinical adoption of DNA vaccines has been a decrease in the platform's immunogenicity when moving to larger animals [Liu, M. A. and J. B. Ulmer, Human clinical trials of plasmid DNA vaccines. Adv Genet, 2005. 55: p. 25-40]. Recent technological advances in the engineering of DNA vaccine immunogen, such has codon optimization, RNA optimization and the addition of immunoglobulin leader sequences have improved expression and immunogenicity of DNA vaccines [Andre, S., et al., Increased immune response elicited by DNA vaccination with a synthetic gp120 sequence with optimized codon usage. J Virol, 1998. 72(2): p. 1497-503; Deml, L., et al., Multiple effects of codon usage optimization on expression and immunogenicity of DNA candidate vaccines encoding the human immunodeficiency virus type 1 Gag protein. J Virol, 2001. 75(22): p. 10991-1001; Laddy, D. J., et al., Immunogenicity of novel consensus-based DNA vaccines against avian influenza. Vaccine, 2007. 25(16): p. 2984-9; Frelin, L., et al., Codon optimization and mRNA amplification effectively enhances the immunogenicity of the hepatitis C virus nonstructural 3/4A gene. Gene Ther, 2004. 11(6): p. 522-33], as well as, recently developed technology in plasmid delivery systems such as electroporation [Hirao, L. A., et al., Intradermal/subcutaneous immunization by electroporation improves plasmid vaccine delivery and potency in pigs and rhesus macaques. Vaccine, 2008. 26(3): p. 440-8; Luckay, A., et al., Effect of plasmid DNA vaccine design and in vivo electroporation on the resulting vaccine-specific immune responses in rhesus macaques. J Virol, 2007. 81(10): p. 5257-69; Ahlen, G., et al., In vivo electroporation enhances the immunogenicity of hepatitis C virus nonstructural 3/4A DNA by increased local DNA uptake, protein expression, inflammation, and infiltration of CD3+ T cells. J Immunol, 2007. 179(7): p. 4741-53]. In addition, studies have suggested that the use of consensus immunogens can be able to increase the breadth of the cellular immune response as compared to native antigens alone [Yan, J., et al., Enhanced cellular immune responses elicited by an engineered HIV-1 subtype B consensus-based envelope DNA vaccine. Mol Ther, 2007. 15(2): p. 411-21; Rolland, M., et al., Reconstruction and function of ancestral center-of-tree human immunodeficiency virus type 1 proteins. J Virol, 2007. 81(16): p. 8507-14].
One method for delivering nucleic acid sequences such as plasmid DNA is the electroporation (EP) technique. The technique has been used in human clinical trials to deliver anti-cancer drugs, such as bleomycin, and in many preclinical studies on a large number of animal species.
There remains a need for an immunogenic influenza consensus hemagglutinin protein, for nucleic acid constructs that encode such a protein and for compositions useful to induce immune responses against multiple strains of influenza. There remains a need for effective vaccines against influenza that are economical and effective across numerous influenza subtypes for treating individuals.